1. Field of the Invention
This invention relates generally to the field of manufacturing plastic containers through the blow molding process. More specifically, this invention relates to an improved hot-fill type blow molded plastic container that exhibits improved resistance to deformation as a result of the considerable heat and pressure stress that is applied thereto during and after the nitrogen dosing type hot-fill process, and to processes and materials for manufacturing such a container.
2. Description of the Related Technology
Containers made of biaxially oriented or bioriented polyethylene terephthalate (PET) are in wide use throughout the world for packaging carbonated and non-carbonated beverages and other liquids. Biaxially oriented PET has good mechanical strength, a good appearance, and forms an effective barrier to the gases contained in the liquids and to the oxygen in the air, thus providing good protection against oxidation.
Perishable food and beverage products such as fruit juices are typically filled at elevated temperatures, such as 180 to 190 degrees Fahrenheit, under variable pressure conditions into specially designed PET containers in what is conventionally referred to as the hot-fill process. Container designs that are intended for use with this process are referred to as hot fill type containers. After filling, the containers are sealed by the application of a closure, preventing mass transfer into and out of the container. As the product within the containers cools, the volume that is occupied by the product decreases, thereby inducing a partial vacuum within the container that exerts an inward force upon the sidewall of the container.
The design of hot fill type containers is heavily influenced by the necessity of managing this shrinkage during cooling. Typically, the shrinkage has most commonly been accommodated by molding one or more concave vacuum panel areas into the sidewall of the container that are designed to deflect inwardly as the product cools. By substantially limiting the deformation to the vacuum panel areas, unwanted distortion of other portions of the container is prevented. In the manufacture of such containers, it is often desirable to have relatively more plastic material flow during the molding process to those areas of the container sidewall that are designed to remain rigid, and relatively less to those areas that are designed to flex. An optimal distribution of the plastic material will ensure the desired strength and flexibility characteristics for the container while avoiding waste of material.
One type of hot-fill technology that is currently under development is known as the nitrogen dosing type hot-fill process. The nitrogen dosing type hot-fill process involves injecting a dose of liquid nitrogen into the container during the hot-fill process. The liquid nitrogen gasifies, pressuring the container after application of the closure to an initial elevated pressure, which is typically on the order of about 20-25 psi. As the container cools, this pressure differential between the inside and the outside of the container will reduce itself to a slight internal overpressure. The initial pressurization and subsequent pressure adjustment, in conjunction with the heat that is inherent to the hot-fill process, places a great deal of stress on the walls of the container. Since, unlike the conventional hot-fill process, the pressure is positive, the stress that is placed on the container is different than the stress that is normally applied during a hot-fill procedure in which no nitrogen dosing is used. Conventional container designs that have worked well with the conventional hot-fill process tend to unexpectedly deform and/or fail under the overpressurization that is inherent to the nitrogen dosing process.
Typically, a blow molded PET container includes a threaded finish portion, a neck portion, a main body portion, a base portion that is either a champagne-type base, a footed base or a modified champagne-type base that has some of the characteristics of a footed base, and what is known as a heel portion connecting the main body portion to the base portion. It has been determined by the inventor that the heat and stress applied to the sidewall of the container, and particularly to the heel portion, during the nitrogen dosing hot-fill process is instrumental in causing unwanted permanent deformation of the heel portion and sidewall of the container. In designing such containers, the diameter of the base portion is normally limited to that which is needed to provide a stable contact ring for supporting the container on a flat surface. By minimizing the size of the base portion, material is conserved. At the same time, the diameter of the main body portion needs to be maximized in order to provide the required total container volume. The greater the differential between the sidewall diameter of the main body portion and the outer diameter of the contact ring of the base portion, the steeper the inclination of the heel portion. The inventor has determined that the inclination of the heel portion, and particularly the lower end of the heel portion, is material to the amount of deformation that takes place as a result of the overpressured environment within the container as a result of the nitrogen dosing process.
In forming certain types of plastic containers from a preform, it is known to utilize a preform that has a thickened sidewall portion toward the closed end of the preform in order to provide additional material that is designed to flow into the container base, usually a footed base, during molding. However, this procedure is not known in the manufacture of hot-fill type containers or nitrogen dosing type hot fill containers, which are considered separate technical areas of container manufacturing because of the different design requirements and characteristics of such containers.
A need exists in this area of technology for an improved hot-fill type container that exhibits an improved resistance to deformation during the hot-fill process, and particularly during the nitrogen dosing hot-fill process, as well as for an improved process of manufacturing such a container.